This study is primarily designed to assess the dose response, dose interval, efficacy and safety of three once daily (QD) doses (100mcg, 400mcg and 800mcg) and three twice daily (BID) doses (100mcg, 200mcg and 400mcg,) of GSK961081 administered via DISKUS™ for 28 days in subjects with moderate/severe COPD versus placebo. Salmeterol 50mcg BID is included in the study as an active comparator.